Method for coating metal strips

ABSTRACT

A PROCESS FOR APPLYING A COATING TO A METAL STRIP WHEREBY THE METAL STRIP IS FIRST PRE-TREATED TO CONDITION IT FOR APPLICATION OF THE COATING, AFTER WHICH THE COATING IS APPLIED TO EITHER ONE OR BOTH SIDES OF THE STRIP. THEREAFTER, THE COATED STRIP PASSES THROUGH A HEATING STATION WHERE IT IS HEATED BY MEANS OF ELECTROMAGNETIC INDUCTION. A HOOD SURROUNDS THE INDUCTION HEATING MEANS SO THAT THE VAPORIZED SOLVENTS AND OTHER FUMES ARE PREVENTED FROM PASSING INTO THE ATMOSPHERE AND OVER THE COATED METAL STRIP DOWNSTREAM OF THE HEATING STATION. A HOOD IS PROVIDED WITH CONDENSING MEANS TO CONDENSE THE VAPORIZED SOLVENTS AND TO PERMIT RECOVERY OF THE CONDENSED SOLVENTS, AND IS ALSO PROVIDED WITH AN AFTERBURNER TO BURN UP ANY UNCONDENSED VAPORS OR GASES TO PREVENT CONTAMINATION OF THE ATMOSPHERE. A SERIES OF COOLED ROLLERS ARE PROVIDED DOWNSTREAM OF THE HEATING MEANS TO NOT ONLY ACT AS CONVEYORS FOR THE STRIP, BUT ALSO AS PROGESSIVE COOLING MEANS TO COMPLETE THE CURE OF THE COATING.

April 1971 LE ROY o. swARTz 3,576,664

METHOD FOR COATING METAL STRIPS Original Filed Sept. 10, 1968 INVENTOR. LEROYO. SWAR'TZ ATTORNEY United States Patent 3,576,664 METHOD FOR COATING METAL STRIPS Le Roy 0. Swartz, Cornwells Heights, Pa., assignor to Cornwells Metal Finishing Company, Inc., 'Cornwells Heights, Pa.

Original application Sept. 10, 1968, Ser. No. 758,896. Divided and this application June 17, 1970, Ser. No. 47,075

Int. Cl. B44d 1/46; F26b 3/ 34; H05b 5/00 U.S. Cl. 117--93.2 3 Claims ABSTRACT OF THE DISCLOSURE A process for applying a coating to a metal strip whereby the metal strip is first pre-treated to condition it for application of the coating, after which the coating is applied to either one or both sides of the strip. Thereafter, the coated strip passes through a heating station where it is heated by means of electromagnetic induction. A hood surrounds the induction heating means so that the vaporized solvents and other fumes are prevented from passing into the atmosphere and over the coated metal strip downstream of the heating station. A hood is pro vided with condensing means to condense the vaporized solvents and to permit recovery of the condensed solvents, and is also provided with an afterburner to burn up any uncondensed vapors or gases to prevent contamination of the atmosphere. A series of cooled rollers are provided downstream of the heating means to not only act as conveyors for the strip, but also as progressive cooling means to complete the cure of the coating.

This is a division of co-pending application Ser. No. 758,896, filed Sept. 10, 1968.

This invention relates to coating of metal strips with heat-curable paint, and it particularly relates to a process for applying such coating in a continuous process.

It has, heretofor e, been the general practice in the coating of metal strips, constructed of such materials as low carbon steel and the like, to apply the coating to the strip and then pass the strip through a curing oven where heat is applied to effect curing of the strip. Since a temperature higher than a predetermined temperature would cause deteiioration of the coating and melting of the strip itself, it Was necessary to use a temperature substantially lower than the deterioration or decomposition point of the coating and the melting point of the strip in the curing process. This resulted in the necessity of applying the heat over an extended period of time, which, in turn, necessitated the use of very elongated ovens to permit the continuously travelling strip to remain in the heated environment for a sufficient time to effect the required cure of the coating.

There were various disadvantages inherent in the abovedescribed process. One disadvantage resided in the fact that the time involved was unduly extended because of the necessity for the strip to travel the great distance through the elongated oven. Another disadvantage was that the elongated oven required a great deal of space which could otherwise be more efiiciently used. A third disadvantage was that if a breakdown occurred, or if it were necessary to stop the machine for any other reason, the portion of the strip then passing through the oven would remain stationary and would be subject to such an accumulation of heat, that the strip might melt or the coating might decompose or be otherwise deteriorated. Even if the oven were immediately deactivated, the accumulated heat therein would still have this effect since the oven could not be cooled with sufficient rapidity to dissipate such heat.

The substitution of an induction heating means for the elongated oven overcomes the problem of heat accumulation since, as soon as the induction heater is deactivated, the heat stops flowing and there is no accumulated heat because the induction heater is open to atmosphere. Such induction heating means also obviates the necessity of having an elongated heating area and, therefore, saves considerable space.

A further advantage of the use of induction heating means is that a very high temperature can be instantaneously induced. Such temperature may be considerably higher than either the deterioration temperature of the coating or the melting point of the strip, but as long as the coated strip is thereafter immediately cooled below such temperature, there is little likelihood of damage. However, this cooling must be effected before the accumulation of heat causes deterioration and melting. Furthermore, if the heat is permitted to flow into the surrounding area downstream of the heating means, it might still be sufficient to cause deterioration and melting of the coated strip. One manner of avoiding this is to provide a cooling means close to the heating means and to cool the coated strip at the same rate at which it was heat ed. However, such rapid cooling is disadvantageous because the rapid heating and cooling cycles tend to warp and weaken the coated strip, and breakage often occurs as the strip continues along its treating path. The resultant product is also often warped and weakened.

It is an object of the present invention to provide a process for applying a cured coating on a metal strip in a continuous production line wherein an induction heating means with all its advantages is utilized but wherein the coating is effectively cured without any warpage or weakening thereof.

Another object of the present invention is to provide a process of the aforesaid type wherein the cooling means is an integral part of the strip moving means.

Another object of the present invention is to provide a process and product of the aforesaid type wherein the solvents in the coating material are substantially recovered.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following description when read in conjunction with the accompanying drawing wherein:

The single figure in the drawing is a schematic view of a system embodying the present invention.

Referring now in greater detail to the drawing wherein similar reference characters refer to similar parts there is shown a continuous treating system, generally des ignated 10, comprising a low carbon steel strip 12 passing from a coil 14 mounted on an uncoiler 16 of standard construction. The strip 12 passes between a pair of pinch rolls 18 and then through a series of pre-treating operations prior to the coating operation.

The first of these pre-treating operations is performed by the apparatus 20 where sprays 22 of alkali solution are projected against both sides of the strip 12 and the strip is then scrubbed and cleaned by scrubbing rollers 24 while the strip is supported by back-up rollers 26. The strip is then guided by pinch rollers 28 into an apparatus 30 where the strip is rinsed by sprays 32 and smoothed by abrasive wheels 34 while being supported by backup rollers 36. The cleaned and smoothed strip 12 is then guided by pinch rollers 38 into an apparatus 40 where the strip is provided with a chemical coating to condition the strip for the application of the paint, this coating, which is of standard formulation, being applied by means of rollers 42. The strip 12 then passes successively through a rinsing apparatus designated generally as 44, a drying means 46 and an auxiliary drying and leveling means 3 generally designated 48 and 50. The strip 12 then passes around a roller 52 before and after which it is provided with a coating of paint by means of rollers 54 and 56 in the standard manner.

It is to be understood that the above-described pre-treating operations are standard operations which, by themselves, form no part of the present invention, although they are part of the total processing system. Furthermore, other pre-treating operations may be substituted without affecting the actual inventive process of the present invention.

After the paint has been applied as a coating to both sides of the strip 12 by the rollers 54 and 56, the strip passes through an induction heating coil 58. The coil 58 is connected to a source of electrical energy (not shown) which supplies the coil with high frequency electrical oscillations, to create a field of force surrounding the strip and, thereby, induce heat producing eddy currents in the strip. A control unit 60, of standard design is imposed in the circuit to vary the current, and therefore the temperature, as desired. A recorder 62 is also provided in the circuit to record the temperature at any particular in stant in response to a sensing device 63.

The heat produced by the induction coil 58 not only acts to heat each segment of the strip to the desired curing temperature as such segments pass through the coil, but also acts to vaporize the solvent which forms a component of all standand paint compositions. This vaporized solvent would ordinarily then fiow downstream of the path of travel of the strip 12, during which time it would begin to condense in the atmosphere. This condensation, being an exothermic process, would give off the heat of vaporization to the coated strip. Such additional heat would not only prevent immediate cooling of the coated strip but would often actually build up the temperature to a point where it would be sufiicient to cause deterioration of the coating material or melting of the strip itself. Furthermore, the solvent would tend to contaminate not only the atmosphere but the coated strip itself which, by this time, should desirably be free of solvent. In addition, the solvent, itself, is a useful product and, when it is permitted to dissipate into the atmosphere, it becomes an economic loss.

In order to overcome the above-described problems relating to the vaporized solvent, there is provided a hood 64 which overlies the coil 58 and which is provided with a slot through which the strip 12 passes in its course through the system. This hood 64 is provided with de pending box-like portions 65 having openings 66 adjacent the upper end of the coil 58. Condensing coils 67 extend down into the portions 65 and are cooled by cold water flowing thereinto from a source (not shown) through line 68. The water passes from the condensing coils to a sump (not shown) through line 70.

As the vapors are formed, they pass through the openings 66 into the hood 64, a blower 71 drawing the vapors therethrough.

The solvent is condensed by the coils 67 and flows down through the depending portions 65 and through conduits 72 to a recovery tank 74. Any vapors or fumes that are not condensed are drawn up through an exhaust duct 76 by the blower 71 to an afterburner 78 where they are burnt up to prevent any contamination of the atmosphere.

The strip 12 thereafter passes around an internally water-cooled roller 80 provided closely adjacent the hood 64, and then successively around rollers 82 and 84 which are also internally water-cooled. Cold water is supplied to all three rollers 80, 82 and 84 from a source (not shown) through a line 86 and a manifold water supply line 88. Each roller 80, 82 and 84 have water outlets, as indicated at 90, 92 and 94 respectively, which lead to a sump or the like (not shown). The coated strip finally passes over guide rollers 96 to a coil 98 mounted on a standard recoiler 100. The recoiler 100 is powered by a motor (not shown), in the standard manner, to draw the strip from the coil 14 in a continuous manner.

The use of the water-cooled rollers 80, 82 and 84 is important in this invention because they provide a means for gradually cooling the coated strip during the curing stage in a manner to lower the temperature sufficiently to prevent deterioration or melting while not cooling the strip so rapidly as to initiate stresses and strains therein and consequent warpage and weakness. This is, of course, aided by the provision of the hood 64 which prevents much of the residual heat built up by the induction coil 58 from passing over the coated strip in the areas of cooling.

It is to be understood that water used to cool the rollers 80, 82 and 84 may, instead of being passed to a sump, be passed through a heat exchanger of any ordinary design and, thereby, be recycled for repeated use in a closed system. It is also within the scope of the present invention to utilize any other feasible type of cooling means such as air or other cooled gases, vapors or liquids.

The strip 12 may be any feasible material which lends itself to the described process. Low carbon sheet steel has been utilized in the description herein, but other types of material may be used if they possess the desired characteristics and if they can be coated with the desired paint or the like by the use of this process. The coating is usually an organic paint composition including an organic solvent, but any solvent which will volatilize under these conditions without contamination of the coating may be used.

The thickness of the strip and coating material as Well as the type of metal of the strip and the chemical formulations and characteristics of the coating are factors in the heating and cooling cycles and these must be carefully controlled in accordance with such conditions. Generally, the temperature, when using standard type paints, is about from 350 F. to about 1000" The thickness of the strip used in the above described process is about 16 gage and the coating speed is preferably about 300 f.p.m. The thickness of the coating may vary within desirable and feasible limits depending on the type of coating substance used, the speed of movement of the strip and the heat generated by coil 58. The speed may, however, vary from about 250 to about 800 f.p.m., depending to a great extent on the amount of cooling rollers used, their cooling effect and their spacing. In this respect, although three such cooling rollers 80, 82 and 84 are used, the number may decrease or increase in accordance with conditions. Furthermore, these rollers may be made adjustable to vary their positions with respect to the heating means as well as with respect to each other. In such case, if a watercooling system such as shown in the drawing is used, the line 84 may be made flexible or telescopic or may be made adjustable in any other manner that would be within the skill of the art.

It is also to be understood that although the invention has been described as relating to coatings having solvents therein, it is equally well adapted to the evaporation of any liquid vehicle or diluent of which solvents are only one example.

Obviously, many modifications of the present invention are possible in the light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

The invention claimed is:

1. In a method of coating a metal strip, the steps which comprise passing the strip in a continuous path through a coating station where a coating containing a vaporisable liquid diluent is applied to at least one surface of the strip, then through a heating station where heat is induced in each segment of the strip as it passes therethrough by electromagnetic induction, said heat being substantially uniform throughout the passage of the strip through the heating station and being sufiicient to initiate curing of said coating, immediately removing vapor generated by said heat during heating from heat transfer relationship With said metal strip, continuing said heating until generation of vapor ceases and said vapor removing is complete, then progressively cooling each segment of the coated strip as it passes from a point adjacent the heating station to a point remote therefrom.

2. The method of claim 1 wherein said vapor is restrained from flowing downstream of the path of travel of the strip after the strip leaves the heating station.

3. The method of claim 2 wherein the vaporized diluent is condensed immediately after it is removed from heat transfer relationship with said strip and the condensed diluent is removed from the system.

References Cited UNITED STATES PATENTS ALFRED L. LEAVITT, Primary Examiner I. H. NEWSOME, Assistant Examiner US. Cl. X.R. 

